Nuclear reactor fuel with radially varying enrichment

ABSTRACT

The fuel pellets inside the fuel rods of a boiling water reactor are made with a lower enrichment concentration in the axial radial region and a higher concentration near the peripheral radial region to reduce thermal expansion effects upon sudden rises in temperature of the pellet when the reactor power is abruptly increased by a rapid step-wise withdrawal of a control rod. At least rods immediately adjacent the control rods contain pellets of this type, but additional rods may also be of this construction.

FIELD OF THE INVENTION

The invention relates to nuclear fission reactors for turbine steamgeneration, particularly those of the boiling water reactor (BWR) type.

BACKGROUND OF THE INVENTION

In a BWR, a core is immersed in water inside a reactor vessel to heatthe water for generating steam. The core is made up of a plurality offuel assemblies, each of which is a bundle of parallel, regularly-spacedmetal cladding fuel tubes filled with fuel pellets of enrichedfissionable material. The water being heated acts as moderator togenerate thermal neutrons for the fission reaction. Control of thereaction is afforded by means of neutron-absorbing control rodsextending into the core within control rod channels formed by relativelywide gaps between the fuel assemblies and inserted or withdrawnlongitudinally to determine the desired total neutron flux. Typically,the control rods have a cruciform crossection, with four arms extendingwithin the gaps between four adjacent assemblies, which are arranged ina square cross section.

One problem with BWR's is that when a control rod is abruptly moved evena short distance in the withdrawal direction to increase the poweroutput, such as occurs when control movement is made in steps, the waterwhich moves into the space previously occupied by the control rodprovides greatly increased neutron flux for the fuel rods at the outsideof the fuel assemblies which border the control rod channel. This leadsto a rapid temperature rise in the interior bulk of their fuel pellets,with attendant possible damage to their cladding by fission gas releasewhich can cause cladding embrittlement and by known radial thermalexpansion fuel-cladding interaction phenomena. This effect can becountered by reducing the flow of water through the reactor to lower theneutron flux of the entire reactor prior to withdrawing the control rod.However, such measures result in a significant power production loss.

SUMMARY OF THE INVENTION

In accordance with the present invention, the fuel rods located at theoutside of the fuel assemblies and bordering the water in the controlrod channel contain a novel type of "dual enrichment" fuel pellet whichhas a lower concentration of uranium enrichment in its inner radialregion near its longitudinal axis than in its outer radial region remotefrom the axis. With this dual enrichment fuel pellet content, the fueltubes maintain a more uniform and reduced temperature internally andtherefore exhibit less radial thermal expansion when the control rod iswithdrawn. This means that the power need not be reduced prior tocontrol rod withdrawal, resulting in significant avoidance of powerproduction losses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a single fuel assembly and a portionof an adjacent control rod.

FIG. 2 is a schematic representation of a fuel assembly cross-sectionshowing specific fuel rod locations.

FIG. 3 is a schematic cross-sectional view of a fuel pellet with dualenrichment in accordance with the invention.

FIG. 4 is a table showing fuel enrichment combinations for the pellet ofFIG. 3.

FIG. 5 is a graphical representation of the temperature distribution inthe dual enrichment pellet as compared to a single enrichment pellet.

FIG. 6-8 are local power distribution charts for fuel rods of anassembly comprising fuel rods with dual enrichment in accordance withone embodiment of the present invention.

DETAILED DESCRIPTION

A typical BWR fuel assembly consists of a cross-sectionally square arrayof fuel rods held by spacers and surrounded by a control rod channel.The control rods used in a BWR are cruciform in shape and are located ina control rod channel between fuel assemblies. An outline drawing of thecross-section of a single fuel assembly 10 and a control rod 12 in achannel 14 is shown in FIG. 1. When the reactor is operating, some ofthe control rods 12 are inserted, some are partially inserted, and theremainder are withdrawn from the reactor core. The space 14 betweencontrol rod channels becomes filled with unvoided water when the controlrod 12 is withdrawn. The unvoided water supplies a high flux of thermalneutrons to the fuel rods 18 bordering the channel 14.

To produce an acceptable power distribution across the fuel assembly,several different Uranium-235 enrichments are normally used in a BWRfuel design. Also, there may be one or more water-filled rods 18 foradjustment of cross-sectional moderator distribution. The enrichments ofthe fuel rods 16 bordering the channel 14 are typically lower than theaverage in order to compensate for the higher thermal neutron flux. Anexample of a BWR Uranium-235 enrichment design is shown schematically inFIG. 2. In the FIG. 2, the square boxes in the array represent fuel rodlocations. Those boxes outlined in solid lines indicate locationsleaving fuel rods most susceptible to damage resulting from step-wisecontrol rod movement.

The control rods in most boiling water reactors (BWR) are designed tomove step-wise in increments of six inches. Reactor operating experiencewith 8×8 fuel assemblies has shown that if the control rods are movedstep-wise at full power, there is a significant probability of fuelfailure in the higher exposure fuel rods, which are operating at highertemperatures and are located adjacent the control channel.

When the reactor is operating with a control rod 12 inserted, the fuelrods 16 located next to the control rod 12 accumulate less neutronexposure than the assembly average exposure, which is attributable to acontrol blade history effect. When the control rod 12 is withdrawn, thepower is therefore higher in the lower exposure fuel rods.

The present invention provides a reduction in the temperature in a fuelpellet by the provision of dual concentrations of Uranium-235 enrichmentin the radial direction across the fuel pellet. A cross-section of sucha pellet 20 is shown in FIG. 3. By lowering the Uranium-235 enrichmentin axial region 22 of the pellet and increasing the enrichment of thefuel in the peripheral region 24 located next to the cladding tube 26clad, the maximum fuel temperature is reduced without changing therelative power generation in the fuel rod 16. Typical Pellet Uranium-235enrichments for the dual enrichment pellet 20 are compared to singleenrichments in the Table 1 of FIG. 4. The numbers represent percentageof Uranium-235 in the pellet material. A plot of fuel temperature as afunction of pellet radius for both a single enrichment and a dualenrichment pellet design is shown in FIG. 5.

In accordance with this invention, fuel rods containing dual enrichmentfuel pellets are used in place of ones containing single enrichmentpellets in any fuel rods which are susceptible to pellet-claddinginteraction failure. Since the fuel temperature is lower in the dualenrichment pellet design, the control rods can be then moved step-wiseat a higher reactor power, resulting in minimal or no loss of reactorpower production. Typical fuel assembly locations for the fuel rodscontaining dual enrichment pellets are indicated by solid line boxes inFIGS. 6 through 7, which show local power distribution maps, includingcontrol blade history effects for an uncontrolled and a controlled fuelassembly. The fuel rod type designations of FIG. 4 are used. The ratioof the uncontrolled power distribution to the controlled powerdistribution is shown in FIG. 8. Again, the numerical figures representpresent concentration of U-235.

In fuel rods operating at lower relative powers or not located close tothe control rods, the standard single enrichment pellet approach isretained, in order to minimize the fuel costs. The maximum Uranium-235enrichment is higher for the dual enrichment design. By using the singleenrichment design in the high enrichment fuel rods, the requiredUranium-235 enrichments can be kept within the fuel manufacturingcriticality safety limits, which are usually set at around 5%.

While thus far the invention has been presented in terms of fuel pelletswith two different concentrations of enrichment, it is contemplated thatsimilar advantages would be obtained with a graded concentration whichdecreases as the central axis of the pellet is approached. However, sucha pellet design would be difficult to manufacture. Alternatively, thepellet could have more than two different concentric radial regions ofdifferent concentrations. The concentrations and their distributionwould be designed to minimize peak and average cross-sectionaltemperature in the pellet, thereby minimizing radial thermal expansionwith temperature change of the pellet as a whole and reducing therelease of fission products such as cesium and iodine which have anembrittling effect on the cladding and contribute to failure by stresscorrosion cracking as stresses are applied to the cladding during pelletthermal expansion.

It should also be evident that varying enrichment concentration fuelrods would be likely to be well suited for other fuel rod locations in areactor, except for their relatively high cost of manufacture.

We claim:
 1. A nuclear reactor fuel assembly, comprising:a cylindricalfuel cladding tube having a longitudinal axis, and fissionable nuclearfuel inside said tube, said fuel comprising an enriched material, theconcentration of the enriched material being less in the radial regionof the longitudinal axis than in the radial region remote from thelongitudinal axis.